Nonwoven fabrics are useful for a wide variety of applications, including absorbent personal care products, garments, medical applications, and cleaning applications. Nonwoven personal care products include infant care items such as diapers, child care items such as training pants, feminine care items such as sanitary napkins, and adult care items such as incontinence products. Nonwoven garments include protective workwear and medical apparel such as surgical gowns. Other nonwoven medical applications include nonwoven wound dressings and surgical dressings. Cleaning applications for nonwovens include towels and wipes. Still other uses of nonwoven fabrics are well known. The foregoing list is not considered exhaustive.
Various properties of nonwoven fabrics determine the suitability of nonwoven fabrics for different applications. Nonwoven fabrics may be engineered to have different combinations of properties to suit different needs. Variable properties of nonwoven fabrics include liquid-handling properties such as wettability, distribution, and absorbency, strength properties such as tensile strength and tear strength, softness properties, durability properties such as abrasion resistance, and aesthetic properties. The physical shape of a nonwoven fabric also affects the functionality and aesthetic properties of the nonwoven fabric. Nonwoven fabrics are initially made into sheets which, when laid on a flat surface, may have a substantially planar, featureless surface or may have an array of surface features such as apertures or projections, or both. Nonwoven fabrics with apertures or projections are often referred to as three-dimensional or shaped nonwoven fabrics. The present invention relates to three-dimensional or shaped nonwoven fabrics.
The manufacture of nonwoven fabrics is a highlydeveloped art. Generally, nonwoven webs and their manufacture involve forming filaments or fibers and depositing the filaments or fibers on a carrier in such a manner so as to cause the filaments or fibers to overlap or entangle. Depending on the degree of web integrity desired, the filaments or fibers of the web may then be bonded by means such as an adhesive, the application of heat or pressure, or both, sonic bonding techniques, or hydroentangling, or the like. There are several methods of producing fibers or filaments within this general description; however, two commonly used processes are known as spunbonding and meltblowing and the resulting nonwoven fabrics are known as spunbond and meltblown fabrics, respectively. As used herein, polymeric fibers and filaments are referred to generically as polymeric strands. Filaments means continuous strands of material and fibers mean cut or discontinuous strands having a definite length.
Generally described, the process for making spunbond nonwoven fabrics includes extruding thermoplastic material through a spinneret and drawing the extruded material into filaments with a stream of high-velocity air to form a random web on a collecting surface. Such a method is referred to as meltspinning. Spunbond processes are generally defined in numerous patents including, for example, U.S. Pat. No. 4,692,618 to Dorschner, et al.; U.S. Pat. No. 4,340,563 to Appel, et al.; U.S. Pat. No. 3,338,992 to Kinney; U.S. Pat. No. 3,341,394 to Kinney; U.S. Pat. No. 3,502,538 to Levy; U.S. Pat. No. 3,502,763 to Hartmann; U.S. Pat. No. 3,909,009 to Hartmann; U.S. Pat. No. 3,542,615 to Dobo, et al., and Canadian Patent 803,714 to Harmon.
On the other hand, meltblown nonwoven fabrics are made by extruding a thermoplastic material through one or more dies, blowing a high-velocity stream of air past the extrusion dies to generate an air-conveyed meltblown fiber curtain and depositing the curtain of fibers onto a collecting surface to form a random nonwoven web. Meltblowing processes are generally described in numerous publications including, for example, an article titled "Superfine Thermoplastic Fibers" by Wendt in Industrial and Engineering Chemistry, Vol. 48, No. 8, (1956), at pp. 1342-1346, which describes work done at the Naval Research Laboratories in Washington, D.C.; Naval Research Laboratory Report 111437, dated Apr. 15, 1954; U.S. Pat. Nos. 4,041,203; 3,715,251; 3,704,198; 3,676,242; and 3,595,245; and British Specification 1,217,892.
Spunbond and meltblown nonwoven fabrics can usually be distinguished by the diameters and the molecular orientation of the filaments or fibers which form the fabrics. The diameter of spunbond and meltblown filaments or fibers is the average cross-sectional dimension. Spunbond filaments or fibers typically have average diameters greater than 6 microns and often have average diameters in the range of 15 to 40 microns. Meltblown fibers typically have average diameters of less than 6 microns. However, because larger meltblown fibers, having diameters of at least 6 microns may also be produced, molecular orientation can be used to distinguish spunbond and meltblown filaments and fibers of similar diameters. For a given fiber or filament size and polymer, the molecular orientation of a spunbond fiber or filament is typically greater than the molecular orientation of a meltblown fiber. Relative molecular orientation of polymeric fibers or filaments can be determined by measuring the tensile strength and birefringence of fibers or filaments having the same diameter.
Tensile strength of fibers and filaments is a measure of the stress required to stretch the fiber or filament until the fiber or filament breaks. Birefringence numbers are calculated according to the method described in the spring 1991 issue of INDA Journal of Nonwovens Research, (Vol. 3, No. 2, p. 27). The tensile strength and birefringence numbers of polymeric fibers and filaments vary depending on the particular polymer and other factors; however, for a given fiber or filament size and polymer, the tensile strength of a spunbond fiber or filament is typically greater than the tensile strength of a meltblown fiber and the birefringence number of a spunbond fiber or filament is typically greater than the birefringence number of a meltblown fiber.
A number of patents disclose methods for making shaped or three-dimensional nonwoven fabrics. For example, U.S. Pat. No. 5,098,764 to Drelich, et al., discloses a nonwoven yarn-like fabric with a net-like structure having apertures. The fabric is formed by laying a web of staple fibers on a surface having an array of holes and projections and spraying the web with high-pressure water to form apertures in the web and entangle the fibers. U.S. Pat. No. 4,741,941 to Engelbert, et al., discloses nonwoven webs having apertures in the fabric or projections extending from the fabric, or both. In this patent, the nonwoven webs are made by forming meltblown or spunbond webs onto a surface which has apertures or projections, or both. In addition, U.S. Pat. No. 4,488,928 to Alikhan, et al., discloses a nonwoven web with puffed regions formed by passing a preformed web between two open mesh screens and thermally bonding the web.
Despite the prior advances in the art as described above, there is still a need for improved nonwoven fabrics having surface features such as apertures or projections or both and methods for forming such materials.